The present invention relates to methods and apparatus for refrigerating food being transported by a vehicle to successive unloading sites.
Food is typically transported to grocery stores by trucks which carry enough food to supply a plurality of stores. Thus, such a truck stops repeatedly at successive stores, whereupon workers at each store enter the truck and unload selected amounts of food.
Some trucks carry food that must be kept refrigerated. To that end, the truck includes a cooling system, typically a mechanical compressor-based system which also includes an evaporator and a condenser. Such a system is costly and subject to mechanical breakdown which can adversely affect the quality of the food. In addition, the coolant which is circulated in such a system can pollute the atmosphere if it leaks out.
Cooling systems for vehicles are known which utilize liquid nitrogen as a cooling source, either in combination with, or in lieu of, compressor-based systems. However, there is a problem in that a space cooled by the direct introduction of liquid nitrogen will possess an atmosphere containing so much gaseous nitrogen that it may not be breathable, thereby posing a safety risk to workers. Examples of refrigeration vehicles utilizing liquid nitrogen can be found in U.S. Pat. Nos. 4,060,400 and 3,464,222 as well as Soviet Patent No. 1,204,888.
In U.S. Pat. No. 4,060,400 there is disclosed a refrigerator semi-trailer intended for the transportation of frozen food products for short and long distances. For this application the semi-trailer is equipped with an automated cooling apparatus that includes both a compressor-based system and a cryogenic system. The compressor-based system is used for preliminary cooling of the empty trailer interior to a specified level. If further cooling is required in situations where the compressor system is not efficient, liquid nitrogen is injected directly within the refrigerated space. This arrangement possesses the above-described disadvantages associated with compressor-based systems. Also, the liquid nitrogen system cannot be used during loading/unloading of the truck, because there would not be a breathable atmosphere in the truck.
The Soviet Patent No. 1,204,888 describes a transport refrigeration system for delivery of perishable produce, wherein the liquid nitrogen is fed into open trays located along interior side walls of the refrigerator body. The liquid nitrogen evaporates in the trays, causing a cold vapor of gaseous nitrogen to leave the trays and fill the trailer body, thus cooling down the transported cargo. This system lacks safety provisions to ensure a breathable atmosphere, and the capability to safely and efficiently refrigerate with the doors open.
U.S. Pat. No. 3,464,222 discloses a refrigerator vehicle in the form of a railway car utilizing a liquid nitrogen system. The liquid nitrogen is conducted through a spray tube arrangement, and simultaneously through an evaporator arrangement. As the liquid nitrogen flows through the evaporator arrangement, it is converted to gaseous nitrogen which is used to drive a fan for drawing air into the space to achieve environmental control. Thus, as liquid nitrogen is sprayed within the space, air is drawn into the space. However, a serious safety risk would still be present due to the significant amount of gaseous nitrogen occupying the space, if the spraying were to occur while the car is being loaded or unloaded. If such spraying does not take place during loading or unloading, then the car may become excessively heated, placing the food in jeopardy.
One aspect of the present invention involves a method of maintaining a temperature of a refrigeration space of a food transport vehicle, comprising the steps of:
A) cooling the refrigeration space to a preselected temperature by spraying liquid nitrogen within the space;
B) discontinuing step A;
C) conducting liquid nitrogen through a heat exchanger extending across a portal which communicates the space with ambient air; and
D) actuating a ventilator to force air through the portal and the heat exchanger and into the space, whereby the ambient air is cooled in the heat exchanger and displaces gaseous nitrogen from the space to create a cold breathable atmosphere.
In another aspect of the invention, the temperature of the refrigeration space can be initially reduced and maintained during the on-loading of food, by operating the ventilator in reverse, to draw air into the refrigeration space from a cooled food storehouse and circulate that air through the refrigeration space and out through the portal. No nitrogen is expended at this time.
Another aspect of the present invention relates to a refrigeration vehicle for transporting refrigerated food. The vehicle comprises a vehicle body forming a refrigeration space for containing food, a supply of liquid nitrogen on the vehicle, and a portal disposed in a wall of the vehicle for communicating the space with ambient air. A heat exchanger extends across the portal, and a ventilator is provided for forcing ambient air through the portal and the heat exchanger and into the space, whereby the ambient air is cooled in the heat exchanger. A sprayer arrangement is disposed in the space, and a conduit arrangement is provided for conducting liquid nitrogen from the supply to the sprayer arrangement and the heat exchanger. Valving is disposed in the conduit arrangement and is disposed selectively in a first mode for admitting liquid nitrogen to the sprayer arrangement to the exclusion of the heat exchanger, and a second mode for admitting liquid nitrogen to the heat exchanger to the exclusion of the sprayer arrangement.